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1. Variable:
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int a ;
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This is not only a Declaration, but also a Definition.
Since it is a defintion, it is must "Define only once".
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If you write this in a .h file, and included it everywhere...
We know that "#include" means extracted the original file totally here, so the same var int a has been defined as many times as been included, which is invalid.
ps: compile is ok, since it is local, local in the specific cpp file. When complier is working on this file, he dosen‘t know even a bit information the world outside.
So the generated file .o is also local.
However, Link is not ok. For multi definition error of int a.
2.Function:
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void f();
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So you can declare it many times, everywhere. This is just a name. Comile and Link are both ok.
So when you define Variable in a .h file, "static" "extern" "const" ... are needed.
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原文地址：http://www.cnblogs.com/stevenxiu/p/5557440.html
